/**
 * @file
 * This is the IPv4 packet segmentation and reassembly implementation.
 *
 */

/*
 * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 * Author: Jani Monoses <jani@iv.ro>
 *         Simon Goldschmidt
 * original reassembly code by Adam Dunkels <adam@sics.se>
 *
 */

#include "lwip/opt.h"

#if LWIP_IPV4

#include "lwip/ip4_frag.h"
#include "lwip/def.h"
#include "lwip/inet_chksum.h"
#include "lwip/netif.h"
#include "lwip/stats.h"
#include "lwip/icmp.h"

#include <string.h>

#if IP_REASSEMBLY
/**
 * The IP reassembly code currently has the following limitations:
 * - IP header options are not supported
 * - fragments must not overlap (e.g. due to different routes),
 *   currently, overlapping or duplicate fragments are thrown away
 *   if IP_REASS_CHECK_OVERLAP=1 (the default)!
 *
 * @todo: work with IP header options
 */

/** Setting this to 0, you can turn off checking the fragments for overlapping
 * regions. The code gets a little smaller. Only use this if you know that
 * overlapping won't occur on your network! */
#ifndef IP_REASS_CHECK_OVERLAP
#define IP_REASS_CHECK_OVERLAP 1
#endif /* IP_REASS_CHECK_OVERLAP */

/** Set to 0 to prevent freeing the oldest datagram when the reassembly buffer is
 * full (IP_REASS_MAX_PBUFS pbufs are enqueued). The code gets a little smaller.
 * Datagrams will be freed by timeout only. Especially useful when MEMP_NUM_REASSDATA
 * is set to 1, so one datagram can be reassembled at a time, only. */
#ifndef IP_REASS_FREE_OLDEST
#define IP_REASS_FREE_OLDEST 1
#endif /* IP_REASS_FREE_OLDEST */

#define IP_REASS_FLAG_LASTFRAG 0x01

/** This is a helper struct which holds the starting
 * offset and the ending offset of this fragment to
 * easily chain the fragments.
 * It has the same packing requirements as the IP header, since it replaces
 * the IP header in memory in incoming fragments (after copying it) to keep
 * track of the various fragments. (-> If the IP header doesn't need packing,
 * this struct doesn't need packing, too.)
 */
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
struct ip_reass_helper
{
    PACK_STRUCT_FIELD(struct pbuf *next_pbuf);
    PACK_STRUCT_FIELD(u16_t start);
    PACK_STRUCT_FIELD(u16_t end);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif

#define IP_ADDRESSES_AND_ID_MATCH(iphdrA, iphdrB)  \
  (ip4_addr_cmp(&(iphdrA)->src, &(iphdrB)->src) && \
   ip4_addr_cmp(&(iphdrA)->dest, &(iphdrB)->dest) && \
   IPH_ID(iphdrA) == IPH_ID(iphdrB)) ? 1 : 0

/* global variables */
static struct ip_reassdata *reassdatagrams;
static u16_t ip_reass_pbufcount;

/* function prototypes */
static void ip_reass_dequeue_datagram(struct ip_reassdata *ipr, struct ip_reassdata *prev);
static int ip_reass_free_complete_datagram(struct ip_reassdata *ipr, struct ip_reassdata *prev);

/**
 * Reassembly timer base function
 * for both NO_SYS == 0 and 1 (!).
 *
 * Should be called every 1000 msec (defined by IP_TMR_INTERVAL).
 */
void
ip_reass_tmr(void)
{
    struct ip_reassdata *r, *prev = NULL;

    r = reassdatagrams;
    while (r != NULL)
    {
        /* Decrement the timer. Once it reaches 0,
         * clean up the incomplete fragment assembly */
        if (r->timer > 0)
        {
            r->timer--;
            LWIP_DEBUGF(IP_REASS_DEBUG, ("ip_reass_tmr: timer dec %"U16_F"\n", (u16_t)r->timer));
            prev = r;
            r = r->next;
        }
        else
        {
            /* reassembly timed out */
            struct ip_reassdata *tmp;
            LWIP_DEBUGF(IP_REASS_DEBUG, ("ip_reass_tmr: timer timed out\n"));
            tmp = r;
            /* get the next pointer before freeing */
            r = r->next;
            /* free the helper struct and all enqueued pbufs */
            ip_reass_free_complete_datagram(tmp, prev);
        }
    }
}

/**
 * Free a datagram (struct ip_reassdata) and all its pbufs.
 * Updates the total count of enqueued pbufs (ip_reass_pbufcount),
 * SNMP counters and sends an ICMP time exceeded packet.
 *
 * @param ipr datagram to free
 * @param prev the previous datagram in the linked list
 * @return the number of pbufs freed
 */
static int
ip_reass_free_complete_datagram(struct ip_reassdata *ipr, struct ip_reassdata *prev)
{
    u16_t pbufs_freed = 0;
    u16_t clen;
    struct pbuf *p;
    struct ip_reass_helper *iprh;

    LWIP_ASSERT("prev != ipr", prev != ipr);
    if (prev != NULL)
    {
        LWIP_ASSERT("prev->next == ipr", prev->next == ipr);
    }

    MIB2_STATS_INC(mib2.ipreasmfails);
#if LWIP_ICMP
    iprh = (struct ip_reass_helper *)ipr->p->payload;
    if (iprh->start == 0)
    {
        /* The first fragment was received, send ICMP time exceeded. */
        /* First, de-queue the first pbuf from r->p. */
        p = ipr->p;
        ipr->p = iprh->next_pbuf;
        /* Then, copy the original header into it. */
        SMEMCPY(p->payload, &ipr->iphdr, IP_HLEN);
        icmp_time_exceeded(p, ICMP_TE_FRAG);
        clen = pbuf_clen(p);
        LWIP_ASSERT("pbufs_freed + clen <= 0xffff", pbufs_freed + clen <= 0xffff);
        pbufs_freed += clen;
        pbuf_free(p);
    }
#endif /* LWIP_ICMP */

    /* First, free all received pbufs.  The individual pbufs need to be released
       separately as they have not yet been chained */
    p = ipr->p;
    while (p != NULL)
    {
        struct pbuf *pcur;
        iprh = (struct ip_reass_helper *)p->payload;
        pcur = p;
        /* get the next pointer before freeing */
        p = iprh->next_pbuf;
        clen = pbuf_clen(pcur);
        LWIP_ASSERT("pbufs_freed + clen <= 0xffff", pbufs_freed + clen <= 0xffff);
        pbufs_freed += clen;
        pbuf_free(pcur);
    }
    /* Then, unchain the struct ip_reassdata from the list and free it. */
    ip_reass_dequeue_datagram(ipr, prev);
    LWIP_ASSERT("ip_reass_pbufcount >= clen", ip_reass_pbufcount >= pbufs_freed);
    ip_reass_pbufcount -= pbufs_freed;

    return pbufs_freed;
}

#if IP_REASS_FREE_OLDEST
/**
 * Free the oldest datagram to make room for enqueueing new fragments.
 * The datagram 'fraghdr' belongs to is not freed!
 *
 * @param fraghdr IP header of the current fragment
 * @param pbufs_needed number of pbufs needed to enqueue
 *        (used for freeing other datagrams if not enough space)
 * @return the number of pbufs freed
 */
static int
ip_reass_remove_oldest_datagram(struct ip_hdr *fraghdr, int pbufs_needed)
{
    /* @todo Can't we simply remove the last datagram in the
     *       linked list behind reassdatagrams?
     */
    struct ip_reassdata *r, *oldest, *prev, *oldest_prev;
    int pbufs_freed = 0, pbufs_freed_current;
    int other_datagrams;

    /* Free datagrams until being allowed to enqueue 'pbufs_needed' pbufs,
     * but don't free the datagram that 'fraghdr' belongs to! */
    do
    {
        oldest = NULL;
        prev = NULL;
        oldest_prev = NULL;
        other_datagrams = 0;
        r = reassdatagrams;
        while (r != NULL)
        {
            if (!IP_ADDRESSES_AND_ID_MATCH(&r->iphdr, fraghdr))
            {
                /* Not the same datagram as fraghdr */
                other_datagrams++;
                if (oldest == NULL)
                {
                    oldest = r;
                    oldest_prev = prev;
                }
                else if (r->timer <= oldest->timer)
                {
                    /* older than the previous oldest */
                    oldest = r;
                    oldest_prev = prev;
                }
            }
            if (r->next != NULL)
            {
                prev = r;
            }
            r = r->next;
        }
        if (oldest != NULL)
        {
            pbufs_freed_current = ip_reass_free_complete_datagram(oldest, oldest_prev);
            pbufs_freed += pbufs_freed_current;
        }
    }
    while ((pbufs_freed < pbufs_needed) && (other_datagrams > 1));
    return pbufs_freed;
}
#endif /* IP_REASS_FREE_OLDEST */

/**
 * Enqueues a new fragment into the fragment queue
 * @param fraghdr points to the new fragments IP hdr
 * @param clen number of pbufs needed to enqueue (used for freeing other datagrams if not enough space)
 * @return A pointer to the queue location into which the fragment was enqueued
 */
static struct ip_reassdata *
ip_reass_enqueue_new_datagram(struct ip_hdr *fraghdr, int clen)
{
    struct ip_reassdata *ipr;
#if ! IP_REASS_FREE_OLDEST
    LWIP_UNUSED_ARG(clen);
#endif

    /* No matching previous fragment found, allocate a new reassdata struct */
    ipr = (struct ip_reassdata *)memp_malloc(MEMP_REASSDATA);
    if (ipr == NULL)
    {
#if IP_REASS_FREE_OLDEST
        if (ip_reass_remove_oldest_datagram(fraghdr, clen) >= clen)
        {
            ipr = (struct ip_reassdata *)memp_malloc(MEMP_REASSDATA);
        }
        if (ipr == NULL)
#endif /* IP_REASS_FREE_OLDEST */
        {
            IPFRAG_STATS_INC(ip_frag.memerr);
            LWIP_DEBUGF(IP_REASS_DEBUG, ("Failed to alloc reassdata struct\n"));
            return NULL;
        }
    }
    memset(ipr, 0, sizeof(struct ip_reassdata));
    ipr->timer = IP_REASS_MAXAGE;

    /* enqueue the new structure to the front of the list */
    ipr->next = reassdatagrams;
    reassdatagrams = ipr;
    /* copy the ip header for later tests and input */
    /* @todo: no ip options supported? */
    SMEMCPY(&(ipr->iphdr), fraghdr, IP_HLEN);
    return ipr;
}

/**
 * Dequeues a datagram from the datagram queue. Doesn't deallocate the pbufs.
 * @param ipr points to the queue entry to dequeue
 */
static void
ip_reass_dequeue_datagram(struct ip_reassdata *ipr, struct ip_reassdata *prev)
{
    /* dequeue the reass struct  */
    if (reassdatagrams == ipr)
    {
        /* it was the first in the list */
        reassdatagrams = ipr->next;
    }
    else
    {
        /* it wasn't the first, so it must have a valid 'prev' */
        LWIP_ASSERT("sanity check linked list", prev != NULL);
        prev->next = ipr->next;
    }

    /* now we can free the ip_reassdata struct */
    memp_free(MEMP_REASSDATA, ipr);
}

/**
 * Chain a new pbuf into the pbuf list that composes the datagram.  The pbuf list
 * will grow over time as  new pbufs are rx.
 * Also checks that the datagram passes basic continuity checks (if the last
 * fragment was received at least once).
 * @param ipr points to the reassembly state
 * @param new_p points to the pbuf for the current fragment
 * @return 0 if invalid, >0 otherwise
 */
static int
ip_reass_chain_frag_into_datagram_and_validate(struct ip_reassdata *ipr, struct pbuf *new_p)
{
    struct ip_reass_helper *iprh, *iprh_tmp, *iprh_prev = NULL;
    struct pbuf *q;
    u16_t offset, len;
    struct ip_hdr *fraghdr;
    int valid = 1;

    /* Extract length and fragment offset from current fragment */
    fraghdr = (struct ip_hdr *)new_p->payload;
    len = lwip_ntohs(IPH_LEN(fraghdr)) - IPH_HL(fraghdr) * 4;
    offset = (lwip_ntohs(IPH_OFFSET(fraghdr)) & IP_OFFMASK) * 8;

    /* overwrite the fragment's ip header from the pbuf with our helper struct,
     * and setup the embedded helper structure. */
    /* make sure the struct ip_reass_helper fits into the IP header */
    LWIP_ASSERT("sizeof(struct ip_reass_helper) <= IP_HLEN",
                sizeof(struct ip_reass_helper) <= IP_HLEN);
    iprh = (struct ip_reass_helper *)new_p->payload;
    iprh->next_pbuf = NULL;
    iprh->start = offset;
    iprh->end = offset + len;

    /* Iterate through until we either get to the end of the list (append),
     * or we find one with a larger offset (insert). */
    for (q = ipr->p; q != NULL;)
    {
        iprh_tmp = (struct ip_reass_helper *)q->payload;
        if (iprh->start < iprh_tmp->start)
        {
            /* the new pbuf should be inserted before this */
            iprh->next_pbuf = q;
            if (iprh_prev != NULL)
            {
                /* not the fragment with the lowest offset */
#if IP_REASS_CHECK_OVERLAP
                if ((iprh->start < iprh_prev->end) || (iprh->end > iprh_tmp->start))
                {
                    /* fragment overlaps with previous or following, throw away */
                    goto freepbuf;
                }
#endif /* IP_REASS_CHECK_OVERLAP */
                iprh_prev->next_pbuf = new_p;
            }
            else
            {
                /* fragment with the lowest offset */
                ipr->p = new_p;
            }
            break;
        }
        else if (iprh->start == iprh_tmp->start)
        {
            /* received the same datagram twice: no need to keep the datagram */
            goto freepbuf;
#if IP_REASS_CHECK_OVERLAP
        }
        else if (iprh->start < iprh_tmp->end)
        {
            /* overlap: no need to keep the new datagram */
            goto freepbuf;
#endif /* IP_REASS_CHECK_OVERLAP */
        }
        else
        {
            /* Check if the fragments received so far have no holes. */
            if (iprh_prev != NULL)
            {
                if (iprh_prev->end != iprh_tmp->start)
                {
                    /* There is a fragment missing between the current
                     * and the previous fragment */
                    valid = 0;
                }
            }
        }
        q = iprh_tmp->next_pbuf;
        iprh_prev = iprh_tmp;
    }

    /* If q is NULL, then we made it to the end of the list. Determine what to do now */
    if (q == NULL)
    {
        if (iprh_prev != NULL)
        {
            /* this is (for now), the fragment with the highest offset:
             * chain it to the last fragment */
#if IP_REASS_CHECK_OVERLAP
            LWIP_ASSERT("check fragments don't overlap", iprh_prev->end <= iprh->start);
#endif /* IP_REASS_CHECK_OVERLAP */
            iprh_prev->next_pbuf = new_p;
            if (iprh_prev->end != iprh->start)
            {
                valid = 0;
            }
        }
        else
        {
#if IP_REASS_CHECK_OVERLAP
            LWIP_ASSERT("no previous fragment, this must be the first fragment!",
                        ipr->p == NULL);
#endif /* IP_REASS_CHECK_OVERLAP */
            /* this is the first fragment we ever received for this ip datagram */
            ipr->p = new_p;
        }
    }

    /* At this point, the validation part begins: */
    /* If we already received the last fragment */
    if ((ipr->flags & IP_REASS_FLAG_LASTFRAG) != 0)
    {
        /* and had no holes so far */
        if (valid)
        {
            /* then check if the rest of the fragments is here */
            /* Check if the queue starts with the first datagram */
            if ((ipr->p == NULL) || (((struct ip_reass_helper *)ipr->p->payload)->start != 0))
            {
                valid = 0;
            }
            else
            {
                /* and check that there are no holes after this datagram */
                iprh_prev = iprh;
                q = iprh->next_pbuf;
                while (q != NULL)
                {
                    iprh = (struct ip_reass_helper *)q->payload;
                    if (iprh_prev->end != iprh->start)
                    {
                        valid = 0;
                        break;
                    }
                    iprh_prev = iprh;
                    q = iprh->next_pbuf;
                }
                /* if still valid, all fragments are received
                 * (because to the MF==0 already arrived */
                if (valid)
                {
                    LWIP_ASSERT("sanity check", ipr->p != NULL);
                    LWIP_ASSERT("sanity check",
                                ((struct ip_reass_helper *)ipr->p->payload) != iprh);
                    LWIP_ASSERT("validate_datagram:next_pbuf!=NULL",
                                iprh->next_pbuf == NULL);
                    LWIP_ASSERT("validate_datagram:datagram end!=datagram len",
                                iprh->end == ipr->datagram_len);
                }
            }
        }
        /* If valid is 0 here, there are some fragments missing in the middle
         * (since MF == 0 has already arrived). Such datagrams simply time out if
         * no more fragments are received... */
        return valid;
    }
    /* If we come here, not all fragments were received, yet! */
    return 0; /* not yet valid! */
#if IP_REASS_CHECK_OVERLAP
freepbuf:
    ip_reass_pbufcount -= pbuf_clen(new_p);
    pbuf_free(new_p);
    return 0;
#endif /* IP_REASS_CHECK_OVERLAP */
}

/**
 * Reassembles incoming IP fragments into an IP datagram.
 *
 * @param p points to a pbuf chain of the fragment
 * @return NULL if reassembly is incomplete, ? otherwise
 */
struct pbuf *
ip4_reass(struct pbuf *p)
{
    struct pbuf *r;
    struct ip_hdr *fraghdr;
    struct ip_reassdata *ipr;
    struct ip_reass_helper *iprh;
    u16_t offset, len, clen;

    IPFRAG_STATS_INC(ip_frag.recv);
    MIB2_STATS_INC(mib2.ipreasmreqds);

    fraghdr = (struct ip_hdr *)p->payload;

    if ((IPH_HL(fraghdr) * 4) != IP_HLEN)
    {
        LWIP_DEBUGF(IP_REASS_DEBUG, ("ip4_reass: IP options currently not supported!\n"));
        IPFRAG_STATS_INC(ip_frag.err);
        goto nullreturn;
    }

    offset = (lwip_ntohs(IPH_OFFSET(fraghdr)) & IP_OFFMASK) * 8;
    len = lwip_ntohs(IPH_LEN(fraghdr)) - IPH_HL(fraghdr) * 4;

    /* Check if we are allowed to enqueue more datagrams. */
    clen = pbuf_clen(p);
    if ((ip_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS)
    {
#if IP_REASS_FREE_OLDEST
        if (!ip_reass_remove_oldest_datagram(fraghdr, clen) ||
                ((ip_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS))
#endif /* IP_REASS_FREE_OLDEST */
        {
            /* No datagram could be freed and still too many pbufs enqueued */
            LWIP_DEBUGF(IP_REASS_DEBUG, ("ip4_reass: Overflow condition: pbufct=%d, clen=%d, MAX=%d\n",
                                         ip_reass_pbufcount, clen, IP_REASS_MAX_PBUFS));
            IPFRAG_STATS_INC(ip_frag.memerr);
            /* @todo: send ICMP time exceeded here? */
            /* drop this pbuf */
            goto nullreturn;
        }
    }

    /* Look for the datagram the fragment belongs to in the current datagram queue,
     * remembering the previous in the queue for later dequeueing. */
    for (ipr = reassdatagrams; ipr != NULL; ipr = ipr->next)
    {
        /* Check if the incoming fragment matches the one currently present
           in the reassembly buffer. If so, we proceed with copying the
           fragment into the buffer. */
        if (IP_ADDRESSES_AND_ID_MATCH(&ipr->iphdr, fraghdr))
        {
            LWIP_DEBUGF(IP_REASS_DEBUG, ("ip4_reass: matching previous fragment ID=%"X16_F"\n",
                                         lwip_ntohs(IPH_ID(fraghdr))));
            IPFRAG_STATS_INC(ip_frag.cachehit);
            break;
        }
    }

    if (ipr == NULL)
    {
        /* Enqueue a new datagram into the datagram queue */
        ipr = ip_reass_enqueue_new_datagram(fraghdr, clen);
        /* Bail if unable to enqueue */
        if (ipr == NULL)
        {
            goto nullreturn;
        }
    }
    else
    {
        if (((lwip_ntohs(IPH_OFFSET(fraghdr)) & IP_OFFMASK) == 0) &&
                ((lwip_ntohs(IPH_OFFSET(&ipr->iphdr)) & IP_OFFMASK) != 0))
        {
            /* ipr->iphdr is not the header from the first fragment, but fraghdr is
             * -> copy fraghdr into ipr->iphdr since we want to have the header
             * of the first fragment (for ICMP time exceeded and later, for copying
             * all options, if supported)*/
            SMEMCPY(&ipr->iphdr, fraghdr, IP_HLEN);
        }
    }
    /* Track the current number of pbufs current 'in-flight', in order to limit
    the number of fragments that may be enqueued at any one time */
    ip_reass_pbufcount += clen;

    /* At this point, we have either created a new entry or pointing
     * to an existing one */

    /* check for 'no more fragments', and update queue entry*/
    if ((IPH_OFFSET(fraghdr) & PP_NTOHS(IP_MF)) == 0)
    {
        ipr->flags |= IP_REASS_FLAG_LASTFRAG;
        ipr->datagram_len = offset + len;
        LWIP_DEBUGF(IP_REASS_DEBUG,
                    ("ip4_reass: last fragment seen, total len %"S16_F"\n",
                     ipr->datagram_len));
    }
    /* find the right place to insert this pbuf */
    /* @todo: trim pbufs if fragments are overlapping */
    if (ip_reass_chain_frag_into_datagram_and_validate(ipr, p))
    {
        struct ip_reassdata *ipr_prev;
        /* the totally last fragment (flag more fragments = 0) was received at least
         * once AND all fragments are received */
        ipr->datagram_len += IP_HLEN;

        /* save the second pbuf before copying the header over the pointer */
        r = ((struct ip_reass_helper *)ipr->p->payload)->next_pbuf;

        /* copy the original ip header back to the first pbuf */
        fraghdr = (struct ip_hdr *)(ipr->p->payload);
        SMEMCPY(fraghdr, &ipr->iphdr, IP_HLEN);
        IPH_LEN_SET(fraghdr, lwip_htons(ipr->datagram_len));
        IPH_OFFSET_SET(fraghdr, 0);
        IPH_CHKSUM_SET(fraghdr, 0);
        /* @todo: do we need to set/calculate the correct checksum? */
#if CHECKSUM_GEN_IP
        IF__NETIF_CHECKSUM_ENABLED(ip_current_input_netif(), NETIF_CHECKSUM_GEN_IP)
        {
            IPH_CHKSUM_SET(fraghdr, inet_chksum(fraghdr, IP_HLEN));
        }
#endif /* CHECKSUM_GEN_IP */

        p = ipr->p;

        /* chain together the pbufs contained within the reass_data list. */
        while (r != NULL)
        {
            iprh = (struct ip_reass_helper *)r->payload;

            /* hide the ip header for every succeeding fragment */
            pbuf_header(r, -IP_HLEN);
            pbuf_cat(p, r);
            r = iprh->next_pbuf;
        }

        /* find the previous entry in the linked list */
        if (ipr == reassdatagrams)
        {
            ipr_prev = NULL;
        }
        else
        {
            for (ipr_prev = reassdatagrams; ipr_prev != NULL; ipr_prev = ipr_prev->next)
            {
                if (ipr_prev->next == ipr)
                {
                    break;
                }
            }
        }

        /* release the sources allocate for the fragment queue entry */
        ip_reass_dequeue_datagram(ipr, ipr_prev);

        /* and adjust the number of pbufs currently queued for reassembly. */
        ip_reass_pbufcount -= pbuf_clen(p);

        MIB2_STATS_INC(mib2.ipreasmoks);

        /* Return the pbuf chain */
        return p;
    }
    /* the datagram is not (yet?) reassembled completely */
    LWIP_DEBUGF(IP_REASS_DEBUG, ("ip_reass_pbufcount: %d out\n", ip_reass_pbufcount));
    return NULL;

nullreturn:
    LWIP_DEBUGF(IP_REASS_DEBUG, ("ip4_reass: nullreturn\n"));
    IPFRAG_STATS_INC(ip_frag.drop);
    pbuf_free(p);
    return NULL;
}
#endif /* IP_REASSEMBLY */

#if IP_FRAG
#if !LWIP_NETIF_TX_SINGLE_PBUF
/** Allocate a new struct pbuf_custom_ref */
static struct pbuf_custom_ref *
ip_frag_alloc_pbuf_custom_ref(void)
{
    return (struct pbuf_custom_ref *)memp_malloc(MEMP_FRAG_PBUF);
}

/** Free a struct pbuf_custom_ref */
static void
ip_frag_free_pbuf_custom_ref(struct pbuf_custom_ref *p)
{
    LWIP_ASSERT("p != NULL", p != NULL);
    memp_free(MEMP_FRAG_PBUF, p);
}

/** Free-callback function to free a 'struct pbuf_custom_ref', called by
 * pbuf_free. */
static void
ipfrag_free_pbuf_custom(struct pbuf *p)
{
    struct pbuf_custom_ref *pcr = (struct pbuf_custom_ref *)p;
    LWIP_ASSERT("pcr != NULL", pcr != NULL);
    LWIP_ASSERT("pcr == p", (void *)pcr == (void *)p);
    if (pcr->original != NULL)
    {
        pbuf_free(pcr->original);
    }
    ip_frag_free_pbuf_custom_ref(pcr);
}
#endif /* !LWIP_NETIF_TX_SINGLE_PBUF */

/**
 * Fragment an IP datagram if too large for the netif.
 *
 * Chop the datagram in MTU sized chunks and send them in order
 * by pointing PBUF_REFs into p.
 *
 * @param p ip packet to send
 * @param netif the netif on which to send
 * @param dest destination ip address to which to send
 *
 * @return ERR_OK if sent successfully, err_t otherwise
 */
err_t
ip4_frag(struct pbuf *p, struct netif *netif, const ip4_addr_t *dest)
{
    struct pbuf *rambuf;
#if !LWIP_NETIF_TX_SINGLE_PBUF
    struct pbuf *newpbuf;
    u16_t newpbuflen = 0;
    u16_t left_to_copy;
#endif
    struct ip_hdr *original_iphdr;
    struct ip_hdr *iphdr;
    const u16_t nfb = (netif->mtu - IP_HLEN) / 8;
    u16_t left, fragsize;
    u16_t ofo;
    int last;
    u16_t poff = IP_HLEN;
    u16_t tmp;

    original_iphdr = (struct ip_hdr *)p->payload;
    iphdr = original_iphdr;
    LWIP_ERROR("ip4_frag() does not support IP options", IPH_HL(iphdr) * 4 == IP_HLEN, return ERR_VAL);

    /* Save original offset */
    tmp = lwip_ntohs(IPH_OFFSET(iphdr));
    ofo = tmp & IP_OFFMASK;
    LWIP_ERROR("ip_frag(): MF already set", (tmp & IP_MF) == 0, return ERR_VAL);

    left = p->tot_len - IP_HLEN;

    while (left)
    {
        /* Fill this fragment */
        fragsize = LWIP_MIN(left, nfb * 8);

#if LWIP_NETIF_TX_SINGLE_PBUF
        rambuf = pbuf_alloc(PBUF_IP, fragsize, PBUF_RAM);
        if (rambuf == NULL)
        {
            goto memerr;
        }
        LWIP_ASSERT("this needs a pbuf in one piece!",
                    (rambuf->len == rambuf->tot_len) && (rambuf->next == NULL));
        poff += pbuf_copy_partial(p, rambuf->payload, fragsize, poff);
        /* make room for the IP header */
        if (pbuf_header(rambuf, IP_HLEN))
        {
            pbuf_free(rambuf);
            goto memerr;
        }
        /* fill in the IP header */
        SMEMCPY(rambuf->payload, original_iphdr, IP_HLEN);
        iphdr = (struct ip_hdr *)rambuf->payload;
#else /* LWIP_NETIF_TX_SINGLE_PBUF */
        /* When not using a static buffer, create a chain of pbufs.
         * The first will be a PBUF_RAM holding the link and IP header.
         * The rest will be PBUF_REFs mirroring the pbuf chain to be fragged,
         * but limited to the size of an mtu.
         */
        rambuf = pbuf_alloc(PBUF_LINK, IP_HLEN, PBUF_RAM);
        if (rambuf == NULL)
        {
            goto memerr;
        }
        LWIP_ASSERT("this needs a pbuf in one piece!",
                    (p->len >= (IP_HLEN)));
        SMEMCPY(rambuf->payload, original_iphdr, IP_HLEN);
        iphdr = (struct ip_hdr *)rambuf->payload;

        left_to_copy = fragsize;
        while (left_to_copy)
        {
            struct pbuf_custom_ref *pcr;
            u16_t plen = p->len - poff;
            newpbuflen = LWIP_MIN(left_to_copy, plen);
            /* Is this pbuf already empty? */
            if (!newpbuflen)
            {
                poff = 0;
                p = p->next;
                continue;
            }
            pcr = ip_frag_alloc_pbuf_custom_ref();
            if (pcr == NULL)
            {
                pbuf_free(rambuf);
                goto memerr;
            }
            /* Mirror this pbuf, although we might not need all of it. */
            newpbuf = pbuf_alloced_custom(PBUF_RAW, newpbuflen, PBUF_REF, &pcr->pc,
                                          (u8_t *)p->payload + poff, newpbuflen);
            if (newpbuf == NULL)
            {
                ip_frag_free_pbuf_custom_ref(pcr);
                pbuf_free(rambuf);
                goto memerr;
            }
            pbuf_ref(p);
            pcr->original = p;
            pcr->pc.custom_free_function = ipfrag_free_pbuf_custom;

            /* Add it to end of rambuf's chain, but using pbuf_cat, not pbuf_chain
             * so that it is removed when pbuf_dechain is later called on rambuf.
             */
            pbuf_cat(rambuf, newpbuf);
            left_to_copy -= newpbuflen;
            if (left_to_copy)
            {
                poff = 0;
                p = p->next;
            }
        }
        poff += newpbuflen;
#endif /* LWIP_NETIF_TX_SINGLE_PBUF */

        /* Correct header */
        last = (left <= netif->mtu - IP_HLEN);

        /* Set new offset and MF flag */
        tmp = (IP_OFFMASK & (ofo));
        if (!last)
        {
            tmp = tmp | IP_MF;
        }
        IPH_OFFSET_SET(iphdr, lwip_htons(tmp));
        IPH_LEN_SET(iphdr, lwip_htons(fragsize + IP_HLEN));
        IPH_CHKSUM_SET(iphdr, 0);
#if CHECKSUM_GEN_IP
        IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_IP)
        {
            IPH_CHKSUM_SET(iphdr, inet_chksum(iphdr, IP_HLEN));
        }
#endif /* CHECKSUM_GEN_IP */

        /* No need for separate header pbuf - we allowed room for it in rambuf
         * when allocated.
         */
        netif->output(netif, rambuf, dest);
        IPFRAG_STATS_INC(ip_frag.xmit);

        /* Unfortunately we can't reuse rambuf - the hardware may still be
         * using the buffer. Instead we free it (and the ensuing chain) and
         * recreate it next time round the loop. If we're lucky the hardware
         * will have already sent the packet, the free will really free, and
         * there will be zero memory penalty.
         */

        pbuf_free(rambuf);
        left -= fragsize;
        ofo += nfb;
    }
    MIB2_STATS_INC(mib2.ipfragoks);
    return ERR_OK;
memerr:
    MIB2_STATS_INC(mib2.ipfragfails);
    return ERR_MEM;
}
#endif /* IP_FRAG */

#endif /* LWIP_IPV4 */
